keymap_editor: Fix updating empty keymap (#40909)

Closes #40898

Release Notes:

- Fixed an issue where attempting to add or update a key binding in the
keymap editor with an empty `keymap.json` file would fail
72c6a74505Ben Kunkle committed on 10/27/2025, 1:56:54 PM· committed by GitHubparent941033e
1 file changedLine totals unavailable